Hypothesis: Prior studies have shown that tight junctions (TJ) proteins are critical for epidermal permeability barrier formation in utero, but the function of TJ and their constituent proteins in postnatal epidermis is unknown. We find that the expression and function of mammalian epidermal TJ change during epidermal development. Therefore, we hypothesize that the TJ and stratum corneum lipid barriers subserve different functions, but are interdependent. Description of proposed work: Our most recent research suggests that TJ and the stratum corneum barrier formed by lipid secretion and processing serve complementary roles in epidermal permeability development. We find that water-impermeant TJ, similar to those in other tight epithelia, are formed first during epidermal development, but these TJ lose their ability to block water and ions as the lipid barrier is formed In this R21 proposal, we will test whether epidermal TJ expression is modified in response to lipid barrier function. Aim #1 will assess how changes in the lipid barrier control TJ expression and function in prenatal and perinatal epidermal development. Aim #2 will test whether TJ expression protects the epidermis from Staphylococcus aureus (SA) invasion. These experiments will form the basis for a later R01 proposal Impact: While many laboratories are studying TJ or the lipid barrier in the epidermis, to date no studies have focused on the relationship between these complementary barriers. Moreover, the relationship between these barriers may have a profound effect on the ability of prenatal, premature, and neonatal skin to resist bacterial infection. Thus, this research is likely to exert a sustained, powerful influence n the research studying epidermal development and responses to bacterial infection.